This invention relates to a leak standard generator suitable for calibrating highly sensitive quantitative detectors. More specifically, this invention relates to a relatively simple leak generator capable of delivering accurate standards at extremely low leak rates.
Many leak standard generators in use today rely upon capillary action for supplying a metered leak standard. As disclosed by Christensen in U.S. Pat. No. 3,760,773, the sample material is typically stored in a housing in liquid form and the material heated to produce a vapor thereof. The high temperature vapor, which is at a correspondingly high pressure, is expanded rapidly through a capillary tube or orifice into a region of lower pressure. By accurately controlling the conditions on both the high and low pressure sides of the capillary device within prescribed parameters, a predictable rate of flow of the sample material through the capillary can be maintained. The equipment needed to control the pressure differential over the capillary, however, is generally costly and difficult to maintain within the generally narrow operating limits. Similarly, the turbulence, which is generally associated with the capillary flashing action, can make it somewhat difficult to achieve accurate flow metering at the low leak rates needed to calibrate truly sensitive detectors. Furthermore, the heated vapor generated in the housing tends to condense on the generally cooler walls of the capillary device. This condensation can have a deleterious effect on the operation of the generator at these low flow rates.
The present leak generator relies upon the diffusion properties exhibited by all gases and vapors to provide for a simple but extremely accurate generator capable of delivering leak standards of a sample material at extremely low leak rates.